Mine tailings are the waste materials which remain at a mine after the commercially-useful ore or other material has been extracted. The tailings comprise finely crushed solids, which are suspended in water to form a slurry, and the tailings are disposed of by pumping the slurry into a suitable impoundment.
The tailings comprise mainly crushed rock, which is harmless as far as possible pollution is concerned, but the tailings generally include also a number of minerals, which do have the potential to pollute the groundwater system.
Sulphide minerals comprise one class of minerals which is often present in tailings, and which has a high potential for pollution. A common such suphide mineral is iron (ferrous) sulphide, or pyrite.
When the tailings do contain pyrite, and if no precautions are taken to prevent it, pollution of the groundwater system occurs as follows.
The tailings impoundment is gradually filled up, and once full, the impoundment is abandoned. As the years go by, gradually the water-table falls to an equilibrium position within the tailings, and the pyrite above the water-table comes into contact with atmospheric oxygen. The pyrite therefore starts to oxidise. The reaction, in the presence of moisture, produces ferrous ions (Fe++), sulphate ions (SO4--), and hydrogen ions (H+).
The water containing these acid-producing ions is displaced down through the body of tailings, as further precipitation infiltrates from above. When this water becomes exposed to the atmosphere (as it inevitably will, sooner or later, when the water enters a stream or lake) the Fe++ in that water oxidises and produces a sulphuric acid solution.
The amount of acidity produced in the groundwater depends of course on the quantity of pyrite above the water table. But in fact, it is not just the pyrite that is exposed to the atmosphere that produces the acidity: even the pyrite that lies submerged below the water table contributes to the acidity problem. The main oxidation reaction that takes place in the exposed pyrite produces ferrous Fe++ ions and sulphate SO4-- ions, which enter the water seeping through the tailings. The ferrous ions in this seepage can then be oxidised further to ferric Fe+++ ions, which also enter the water seeping down through the tailings. Under the low pH conditions generated by pyrite oxidation, the ferric iron is displaced below the water table where the ferric ions come into contact with the as-yet un-oxidised FeS2 lying submerged below the water table.
Below the water table, the ferric ions react with the un-oxidised pyrite, producing yet more ferrous ions and more sulphate ions. The water that enters the groundwater system, through the floor of the impoundment, consequently contains substantial concentrations of ferrous and sulphate ions.
If the whole body of tailings could be kept permanently under water, very little oxidation of the pyrite would take place; but once the upper regions of the body of tailings have become exposed, a large portion of the whole body of tailings contributes to the production of acidity.
The invention is concerned with the problem, as described, of preventing acid-producing ions from entering the groundwater.